1. Field of the Invention
The present invention relates to a projection exposure apparatus which is usable to transfer a mask pattern onto a photosensitive substrate in the lithography step for producing devices including, for example, semiconductor elements, image pickup elements (CCD or the like), liquid crystal display elements, and thin film magnetic heads. In particular, the present invention relates to a projection exposure apparatus based on the use of the liquid immersion method. The present invention also relates to an optical element which is usable for the projection exposure apparatus.
2. Description of the Related Art
In the production of the semiconductor element or the like, the projection exposure apparatus is used to transfer a pattern image of a reticle as a mask via a projection optical system onto each of shot areas on a wafer (or a glass plate or the like) coated with a resist as a photosensitive substrate. Conventionally, the reduction projection type exposure apparatus based on the step-and-repeat system (stepper) has been used as the projection exposure apparatus in many cases. However, recently, the attention is also attracted to the projection exposure apparatus based on the step-and-scan system in which the exposure is performed by synchronously scanning the reticle and the wafer.
As for the resolution of the projection optical system carried on the projection exposure apparatus, as the exposure wavelength to be used is shorter, the resolution becomes higher. Further, as the numerical aperture of the projection optical system is larger, the resolution becomes higher. Therefore, the exposure wavelength, which is used for the projection exposure apparatus, is shortened year by year, and the numerical aperture of the projection optical system is increased as well, as the integrated circuit becomes fine and minute. The exposure wavelength, which is dominantly used at present, is 248 nm based on the KrF excimer laser. However, the exposure wavelength of 193 nm based on the ArF excimer laser, which is shorter than the above, is also practically used.
When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δ are represented by the following expressions respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed. Conventionally, in the projection exposure apparatus, the surface of the wafer is adjusted and matched with the image plane of the projection optical system in the auto-focus manner. For this purpose, it is desirable that the depth of focus δ is wide to some extent. Accordingly, those having been suggested as the method for substantially widening the depth of focus include, for example, the phase shift reticle method, the modified illumination method, and the multilayer resist method.
As described above, in the conventional projection exposure apparatus, the depth of focus is narrowed as the wavelength of the exposure light beam is shortened and the numerical aperture of the projection optical system is increased. In order to respond to the advance of higher integration of the semiconductor integrated circuit, studies have been also made to further shorten the exposure wavelength. However, if such a situation is continued as it is, it is feared that the depth of focus may be too narrowed and the margin may become insufficient during the exposure operation.
In view of the above, the liquid immersion method has been proposed as a method for substantially shortening the exposure wavelength and deepening the depth of focus. In this method, the space between the lower surface of the projection optical system and the surface of the wafer is filled with a liquid such as water or an organic solvent. The resolution is improved and the depth of focus is magnified about n times by utilizing the fact that the wavelength of the exposure light beam in the liquid is 1/n time that in the air (n represents the refractive index of the liquid, which is usually about 1.2 to 1.6).
If it is intended to apply the liquid immersion method to the projection exposure apparatus based on the step-and-repeat system as it is, the liquid leaks out from the space between the projection optical system and the wafer when the wafer is subjected to the stepping movement to the next shot area after the exposure is completed for one shot area. Therefore, inconveniences arise such that the liquid must be supplied again, and it is difficult to recover the leaked liquid as well. If it is intended to apply the liquid immersion method to the projection exposure apparatus based on the step-and-scan system, it is necessary that the space between the projection optical system and the wafer is filled with the liquid during the period in which the wafer is moved as well, because the exposure is performed while moving the wafer. The projection optical system and the liquid make contact with each other. Therefore, there is such a possibility that the end portion of the projection optical system, which is in contact with the liquid, may be corroded by the liquid. The objective lens is installed to the end of the projection optical system. If the objective lens is corroded, it is feared that any desired optical performance cannot be obtained.